| Message ID | cover.1683067198.git.lstoakes@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | mm/gup: disallow GUP writing to file-backed mappings by default | expand |
On 5/2/23 6:51 PM, Lorenzo Stoakes wrote: > Writing to file-backed mappings which require folio dirty tracking using > GUP is a fundamentally broken operation, as kernel write access to GUP > mappings do not adhere to the semantics expected by a file system. > > A GUP caller uses the direct mapping to access the folio, which does not > cause write notify to trigger, nor does it enforce that the caller marks > the folio dirty. > > The problem arises when, after an initial write to the folio, writeback > results in the folio being cleaned and then the caller, via the GUP > interface, writes to the folio again. > > As a result of the use of this secondary, direct, mapping to the folio no > write notify will occur, and if the caller does mark the folio dirty, this > will be done so unexpectedly. > > For example, consider the following scenario:- > > 1. A folio is written to via GUP which write-faults the memory, notifying > the file system and dirtying the folio. > 2. Later, writeback is triggered, resulting in the folio being cleaned and > the PTE being marked read-only. > 3. The GUP caller writes to the folio, as it is mapped read/write via the > direct mapping. > 4. The GUP caller, now done with the page, unpins it and sets it dirty > (though it does not have to). > > This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As > pin_user_pages_fast_only() does not exist, we can rely on a slightly > imperfect whitelisting in the PUP-fast case and fall back to the slow case > should this fail. > > v8: > - Fixed typo writeable -> writable. > - Fixed bug in writable_file_mapping_allowed() - must check combination of > FOLL_PIN AND FOLL_LONGTERM not either/or. > - Updated vma_needs_dirty_tracking() to include write/shared to account for > MAP_PRIVATE mappings. > - Move to open-coding the checks in folio_pin_allowed() so we can > READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to > account for fact we now check flags here. > - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than > anon. Defer to slow path. > - Perform GUP-fast check _after_ the lowest page table level is confirmed to > be stable. > - Updated comments and commit message for final patch as per Jason's > suggestions. Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count)
On 03.05.23 02:31, Matthew Rosato wrote: > On 5/2/23 6:51 PM, Lorenzo Stoakes wrote: >> Writing to file-backed mappings which require folio dirty tracking using >> GUP is a fundamentally broken operation, as kernel write access to GUP >> mappings do not adhere to the semantics expected by a file system. >> >> A GUP caller uses the direct mapping to access the folio, which does not >> cause write notify to trigger, nor does it enforce that the caller marks >> the folio dirty. >> >> The problem arises when, after an initial write to the folio, writeback >> results in the folio being cleaned and then the caller, via the GUP >> interface, writes to the folio again. >> >> As a result of the use of this secondary, direct, mapping to the folio no >> write notify will occur, and if the caller does mark the folio dirty, this >> will be done so unexpectedly. >> >> For example, consider the following scenario:- >> >> 1. A folio is written to via GUP which write-faults the memory, notifying >> the file system and dirtying the folio. >> 2. Later, writeback is triggered, resulting in the folio being cleaned and >> the PTE being marked read-only. >> 3. The GUP caller writes to the folio, as it is mapped read/write via the >> direct mapping. >> 4. The GUP caller, now done with the page, unpins it and sets it dirty >> (though it does not have to). >> >> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As >> pin_user_pages_fast_only() does not exist, we can rely on a slightly >> imperfect whitelisting in the PUP-fast case and fall back to the slow case >> should this fail. >> >> v8: >> - Fixed typo writeable -> writable. >> - Fixed bug in writable_file_mapping_allowed() - must check combination of >> FOLL_PIN AND FOLL_LONGTERM not either/or. >> - Updated vma_needs_dirty_tracking() to include write/shared to account for >> MAP_PRIVATE mappings. >> - Move to open-coding the checks in folio_pin_allowed() so we can >> READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to >> account for fact we now check flags here. >> - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than >> anon. Defer to slow path. >> - Perform GUP-fast check _after_ the lowest page table level is confirmed to >> be stable. >> - Updated comments and commit message for final patch as per Jason's >> suggestions. > > Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count) At least it's consistent now ;) And it might be working as expected ... In v7: * pin_user_pages_fast() succeeded * vfio_pin_pages_remote() failed But also in v7: * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file mappings * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings In v8: * pin_user_pages_fast() succeeds * vfio_pin_pages_remote() succeeds But also in v8: * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file mappings * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings I have to speculate, but ... could it be that you are using a private mapping? In QEMU, unfortunately, the default for memory-backend-file is "share=off" (private) ... for memory-backend-memfd it is "share=on" (shared). The default is stupid ... If you invoke QEMU manually, can you specify "share=on" for the memory-backend-file? I thought libvirt would always default to "share=on" for file mappings (everything else doesn't make much sense) ... but you might have to specify <access mode="shared"/> in addition to <source type="file"/>
On 5/3/23 3:08 AM, David Hildenbrand wrote: > On 03.05.23 02:31, Matthew Rosato wrote: >> On 5/2/23 6:51 PM, Lorenzo Stoakes wrote: >>> Writing to file-backed mappings which require folio dirty tracking using >>> GUP is a fundamentally broken operation, as kernel write access to GUP >>> mappings do not adhere to the semantics expected by a file system. >>> >>> A GUP caller uses the direct mapping to access the folio, which does not >>> cause write notify to trigger, nor does it enforce that the caller marks >>> the folio dirty. >>> >>> The problem arises when, after an initial write to the folio, writeback >>> results in the folio being cleaned and then the caller, via the GUP >>> interface, writes to the folio again. >>> >>> As a result of the use of this secondary, direct, mapping to the folio no >>> write notify will occur, and if the caller does mark the folio dirty, this >>> will be done so unexpectedly. >>> >>> For example, consider the following scenario:- >>> >>> 1. A folio is written to via GUP which write-faults the memory, notifying >>> the file system and dirtying the folio. >>> 2. Later, writeback is triggered, resulting in the folio being cleaned and >>> the PTE being marked read-only. >>> 3. The GUP caller writes to the folio, as it is mapped read/write via the >>> direct mapping. >>> 4. The GUP caller, now done with the page, unpins it and sets it dirty >>> (though it does not have to). >>> >>> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As >>> pin_user_pages_fast_only() does not exist, we can rely on a slightly >>> imperfect whitelisting in the PUP-fast case and fall back to the slow case >>> should this fail. >>> >>> v8: >>> - Fixed typo writeable -> writable. >>> - Fixed bug in writable_file_mapping_allowed() - must check combination of >>> FOLL_PIN AND FOLL_LONGTERM not either/or. >>> - Updated vma_needs_dirty_tracking() to include write/shared to account for >>> MAP_PRIVATE mappings. >>> - Move to open-coding the checks in folio_pin_allowed() so we can >>> READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to >>> account for fact we now check flags here. >>> - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than >>> anon. Defer to slow path. >>> - Perform GUP-fast check _after_ the lowest page table level is confirmed to >>> be stable. >>> - Updated comments and commit message for final patch as per Jason's >>> suggestions. >> >> Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count) > > At least it's consistent now ;) And it might be working as expected ... > > In v7: > * pin_user_pages_fast() succeeded > * vfio_pin_pages_remote() failed > > But also in v7: > * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file > mappings > * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings > > In v8: > * pin_user_pages_fast() succeeds > * vfio_pin_pages_remote() succeeds > > But also in v8: > * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file > mappings > * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings > > > I have to speculate, but ... could it be that you are using a private mapping? > > In QEMU, unfortunately, the default for memory-backend-file is "share=off" (private) ... for memory-backend-memfd it is "share=on" (shared). The default is stupid ... > > If you invoke QEMU manually, can you specify "share=on" for the memory-backend-file? I thought libvirt would always default to "share=on" for file mappings (everything else doesn't make much sense) ... but you might have to specify > <access mode="shared"/> > in addition to > <source type="file"/> > Ah, there we go. Yes, I was using the default of share=off. When I instead specify share=on, now the pins will fail in both cases.
On 03.05.23 13:25, Matthew Rosato wrote: > On 5/3/23 3:08 AM, David Hildenbrand wrote: >> On 03.05.23 02:31, Matthew Rosato wrote: >>> On 5/2/23 6:51 PM, Lorenzo Stoakes wrote: >>>> Writing to file-backed mappings which require folio dirty tracking using >>>> GUP is a fundamentally broken operation, as kernel write access to GUP >>>> mappings do not adhere to the semantics expected by a file system. >>>> >>>> A GUP caller uses the direct mapping to access the folio, which does not >>>> cause write notify to trigger, nor does it enforce that the caller marks >>>> the folio dirty. >>>> >>>> The problem arises when, after an initial write to the folio, writeback >>>> results in the folio being cleaned and then the caller, via the GUP >>>> interface, writes to the folio again. >>>> >>>> As a result of the use of this secondary, direct, mapping to the folio no >>>> write notify will occur, and if the caller does mark the folio dirty, this >>>> will be done so unexpectedly. >>>> >>>> For example, consider the following scenario:- >>>> >>>> 1. A folio is written to via GUP which write-faults the memory, notifying >>>> the file system and dirtying the folio. >>>> 2. Later, writeback is triggered, resulting in the folio being cleaned and >>>> the PTE being marked read-only. >>>> 3. The GUP caller writes to the folio, as it is mapped read/write via the >>>> direct mapping. >>>> 4. The GUP caller, now done with the page, unpins it and sets it dirty >>>> (though it does not have to). >>>> >>>> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As >>>> pin_user_pages_fast_only() does not exist, we can rely on a slightly >>>> imperfect whitelisting in the PUP-fast case and fall back to the slow case >>>> should this fail. >>>> >>>> v8: >>>> - Fixed typo writeable -> writable. >>>> - Fixed bug in writable_file_mapping_allowed() - must check combination of >>>> FOLL_PIN AND FOLL_LONGTERM not either/or. >>>> - Updated vma_needs_dirty_tracking() to include write/shared to account for >>>> MAP_PRIVATE mappings. >>>> - Move to open-coding the checks in folio_pin_allowed() so we can >>>> READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to >>>> account for fact we now check flags here. >>>> - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than >>>> anon. Defer to slow path. >>>> - Perform GUP-fast check _after_ the lowest page table level is confirmed to >>>> be stable. >>>> - Updated comments and commit message for final patch as per Jason's >>>> suggestions. >>> >>> Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count) >> >> At least it's consistent now ;) And it might be working as expected ... >> >> In v7: >> * pin_user_pages_fast() succeeded >> * vfio_pin_pages_remote() failed >> >> But also in v7: >> * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file >> mappings >> * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings >> >> In v8: >> * pin_user_pages_fast() succeeds >> * vfio_pin_pages_remote() succeeds >> >> But also in v8: >> * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file >> mappings >> * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings >> >> >> I have to speculate, but ... could it be that you are using a private mapping? >> >> In QEMU, unfortunately, the default for memory-backend-file is "share=off" (private) ... for memory-backend-memfd it is "share=on" (shared). The default is stupid ... >> >> If you invoke QEMU manually, can you specify "share=on" for the memory-backend-file? I thought libvirt would always default to "share=on" for file mappings (everything else doesn't make much sense) ... but you might have to specify >> <access mode="shared"/> >> in addition to >> <source type="file"/> >> > > Ah, there we go. Yes, I was using the default of share=off. When I instead specify share=on, now the pins will fail in both cases. > Out of curiosity, how does that manifest? I assume the VM is successfully created and as Linux tries initializing and using the device, we get a bunch of errors inside the VM, correct?
On 5/3/23 8:53 AM, David Hildenbrand wrote: > On 03.05.23 13:25, Matthew Rosato wrote: >> On 5/3/23 3:08 AM, David Hildenbrand wrote: >>> On 03.05.23 02:31, Matthew Rosato wrote: >>>> On 5/2/23 6:51 PM, Lorenzo Stoakes wrote: >>>>> Writing to file-backed mappings which require folio dirty tracking using >>>>> GUP is a fundamentally broken operation, as kernel write access to GUP >>>>> mappings do not adhere to the semantics expected by a file system. >>>>> >>>>> A GUP caller uses the direct mapping to access the folio, which does not >>>>> cause write notify to trigger, nor does it enforce that the caller marks >>>>> the folio dirty. >>>>> >>>>> The problem arises when, after an initial write to the folio, writeback >>>>> results in the folio being cleaned and then the caller, via the GUP >>>>> interface, writes to the folio again. >>>>> >>>>> As a result of the use of this secondary, direct, mapping to the folio no >>>>> write notify will occur, and if the caller does mark the folio dirty, this >>>>> will be done so unexpectedly. >>>>> >>>>> For example, consider the following scenario:- >>>>> >>>>> 1. A folio is written to via GUP which write-faults the memory, notifying >>>>> the file system and dirtying the folio. >>>>> 2. Later, writeback is triggered, resulting in the folio being cleaned and >>>>> the PTE being marked read-only. >>>>> 3. The GUP caller writes to the folio, as it is mapped read/write via the >>>>> direct mapping. >>>>> 4. The GUP caller, now done with the page, unpins it and sets it dirty >>>>> (though it does not have to). >>>>> >>>>> This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As >>>>> pin_user_pages_fast_only() does not exist, we can rely on a slightly >>>>> imperfect whitelisting in the PUP-fast case and fall back to the slow case >>>>> should this fail. >>>>> >>>>> v8: >>>>> - Fixed typo writeable -> writable. >>>>> - Fixed bug in writable_file_mapping_allowed() - must check combination of >>>>> FOLL_PIN AND FOLL_LONGTERM not either/or. >>>>> - Updated vma_needs_dirty_tracking() to include write/shared to account for >>>>> MAP_PRIVATE mappings. >>>>> - Move to open-coding the checks in folio_pin_allowed() so we can >>>>> READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to >>>>> account for fact we now check flags here. >>>>> - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than >>>>> anon. Defer to slow path. >>>>> - Perform GUP-fast check _after_ the lowest page table level is confirmed to >>>>> be stable. >>>>> - Updated comments and commit message for final patch as per Jason's >>>>> suggestions. >>>> >>>> Tested again on s390 using QEMU with a memory backend file (on ext4) and vfio-pci -- This time both vfio_pin_pages_remote (which will call pin_user_pages_remote(flags | FOLL_LONGTERM)) and the pin_user_pages_fast(FOLL_WRITE | FOLL_LONGTERM) in kvm_s390_pci_aif_enable are being allowed (e.g. returning positive pin count) >>> >>> At least it's consistent now ;) And it might be working as expected ... >>> >>> In v7: >>> * pin_user_pages_fast() succeeded >>> * vfio_pin_pages_remote() failed >>> >>> But also in v7: >>> * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file >>> mappings >>> * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings >>> >>> In v8: >>> * pin_user_pages_fast() succeeds >>> * vfio_pin_pages_remote() succeeds >>> >>> But also in v8: >>> * GUP-fast allows pinning (anonymous) pages in MAP_PRIVATE file >>> mappings >>> * Ordinary GUP allows pinning pages in MAP_PRIVATE file mappings >>> >>> >>> I have to speculate, but ... could it be that you are using a private mapping? >>> >>> In QEMU, unfortunately, the default for memory-backend-file is "share=off" (private) ... for memory-backend-memfd it is "share=on" (shared). The default is stupid ... >>> >>> If you invoke QEMU manually, can you specify "share=on" for the memory-backend-file? I thought libvirt would always default to "share=on" for file mappings (everything else doesn't make much sense) ... but you might have to specify >>> <access mode="shared"/> >>> in addition to >>> <source type="file"/> >>> >> >> Ah, there we go. Yes, I was using the default of share=off. When I instead specify share=on, now the pins will fail in both cases. >> > > Out of curiosity, how does that manifest? > > I assume the VM is successfully created and as Linux tries initializing and using the device, we get a bunch of errors inside the VM, correct? > Yes, that's correct. Which error comes first (an attempt at mapping something via type1 iommu or an attempt to register AEN) depends on the device type and the order of operations of the associated driver. But in either case, you're going to see guest errors associated with that action. mlx5 and ism give up rather quickly and just fail their probe. nvme in the guest is persistent and its actions keep re-attempting to setup AEN by issuing the associated instruction; but the associated blockdev will never show up.
Writing to file-backed mappings which require folio dirty tracking using GUP is a fundamentally broken operation, as kernel write access to GUP mappings do not adhere to the semantics expected by a file system. A GUP caller uses the direct mapping to access the folio, which does not cause write notify to trigger, nor does it enforce that the caller marks the folio dirty. The problem arises when, after an initial write to the folio, writeback results in the folio being cleaned and then the caller, via the GUP interface, writes to the folio again. As a result of the use of this secondary, direct, mapping to the folio no write notify will occur, and if the caller does mark the folio dirty, this will be done so unexpectedly. For example, consider the following scenario:- 1. A folio is written to via GUP which write-faults the memory, notifying the file system and dirtying the folio. 2. Later, writeback is triggered, resulting in the folio being cleaned and the PTE being marked read-only. 3. The GUP caller writes to the folio, as it is mapped read/write via the direct mapping. 4. The GUP caller, now done with the page, unpins it and sets it dirty (though it does not have to). This change updates both the PUP FOLL_LONGTERM slow and fast APIs. As pin_user_pages_fast_only() does not exist, we can rely on a slightly imperfect whitelisting in the PUP-fast case and fall back to the slow case should this fail. v8: - Fixed typo writeable -> writable. - Fixed bug in writable_file_mapping_allowed() - must check combination of FOLL_PIN AND FOLL_LONGTERM not either/or. - Updated vma_needs_dirty_tracking() to include write/shared to account for MAP_PRIVATE mappings. - Move to open-coding the checks in folio_pin_allowed() so we can READ_ONCE() the mapping and avoid unexpected compiler loads. Rename to account for fact we now check flags here. - Disallow mapping == NULL or mapping & PAGE_MAPPING_FLAGS other than anon. Defer to slow path. - Perform GUP-fast check _after_ the lowest page table level is confirmed to be stable. - Updated comments and commit message for final patch as per Jason's suggestions. v7: - Fixed very silly bug in writeable_file_mapping_allowed() inverting the logic. - Removed unnecessary RCU lock code and replaced with adaptation of Peter's idea. - Removed unnecessary open-coded folio_test_anon() in folio_longterm_write_pin_allowed() and restructured to generally permit NULL folio_mapping(). https://lore.kernel.org/all/cover.1683044162.git.lstoakes@gmail.com/ v6: - Rebased on latest mm-unstable as of 28th April 2023. - Add PUP-fast check with handling for rcu-locked TLB shootdown to synchronise correctly. - Split patch series into 3 to make it more digestible. https://lore.kernel.org/all/cover.1682981880.git.lstoakes@gmail.com/ v5: - Rebased on latest mm-unstable as of 25th April 2023. - Some small refactorings suggested by John. - Added an extended description of the problem in the comment around writeable_file_mapping_allowed() for clarity. - Updated commit message as suggested by Mika and John. https://lore.kernel.org/all/6b73e692c2929dc4613af711bdf92e2ec1956a66.1682638385.git.lstoakes@gmail.com/ v4: - Split out vma_needs_dirty_tracking() from vma_wants_writenotify() to reduce duplication and update to use this in the GUP check. Note that both separately check vm_ops_needs_writenotify() as the latter needs to test this before the vm_pgprot_modify() test, resulting in vma_wants_writenotify() checking this twice, however it is such a small check this should not be egregious. https://lore.kernel.org/all/3b92d56f55671a0389252379237703df6e86ea48.1682464032.git.lstoakes@gmail.com/ v3: - Rebased on latest mm-unstable as of 24th April 2023. - Explicitly check whether file system requires folio dirtying. Note that vma_wants_writenotify() could not be used directly as it is very much focused on determining if the PTE r/w should be set (e.g. assuming private mapping does not require it as already set, soft dirty considerations). - Tested code against shmem and hugetlb mappings - confirmed that these are not disallowed by the check. - Eliminate FOLL_ALLOW_BROKEN_FILE_MAPPING flag and instead perform check only for FOLL_LONGTERM pins. - As a result, limit check to internal GUP code. https://lore.kernel.org/all/23c19e27ef0745f6d3125976e047ee0da62569d4.1682406295.git.lstoakes@gmail.com/ v2: - Add accidentally excluded ptrace_access_vm() use of FOLL_ALLOW_BROKEN_FILE_MAPPING. - Tweak commit message. https://lore.kernel.org/all/c8ee7e02d3d4f50bb3e40855c53bda39eec85b7d.1682321768.git.lstoakes@gmail.com/ v1: https://lore.kernel.org/all/f86dc089b460c80805e321747b0898fd1efe93d7.1682168199.git.lstoakes@gmail.com/ Lorenzo Stoakes (3): mm/mmap: separate writenotify and dirty tracking logic mm/gup: disallow FOLL_LONGTERM GUP-nonfast writing to file-backed mappings mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings include/linux/mm.h | 1 + mm/gup.c | 146 ++++++++++++++++++++++++++++++++++++++++++++- mm/mmap.c | 53 ++++++++++++---- 3 files changed, 186 insertions(+), 14 deletions(-) -- 2.40.1